In spectrophotometers for liquid chromatography (LC) a substance whose quantitative presence in a sample is to be determined is injected and dissolved into a suitable liquid carrier solvent, separated in an LC column and flowed through a detector cell which has end windows through which a selected wavelength of ultraviolet or visible light radiation is directed. Radiation exiting from the cell falls on a photodetector having an output recorded by suitable instrumentation which is calibrated to indicate the amount of radiation absorbed by the fluid flowing through the cell. Absorbance is customarily indicated by a graph continuously recorded on a strip chart by a pen recorder. The quantitative presence of a sample substance of interest is determined by measuring the area under the graph peaks which represent the amount of radiation of a particular wavelength that is absorbed, particular materials being identified by particular wavelengths characteristically absorbed by them.
The sensitivity of a spectrophotometer detector cell is a function particularly of the stability of the base line of the graph. The graph base line is established by the absorbance of the solvent used, and will change in relation to any change in the refractive index of the solvent. The refractive index in turn will be changed by a change in the temperature of the solvent in the cell, or by a transient as a test sample reaches the cell. Transients are particularly associated with temporary changes in refractive index on the fluid near the wall of the cell. When the baseline changes, the true peak area cannot be measured accurately and the peaks themselves become less clearly defined and hence difficult to identify and measure with any reliable degree of accuracy. The sensitivity of the instrument is disproportionally reduced by any change in temperatures or composition of the solvent, thus altering its index of refraction and increasing flow noise.
FIG. 1 shows a prior art type of LC detector system 10. A light source 11 provides light to a field lens 12 adjacent to a slit 13 oriented perpendicular to the plane of the drawing. A concave grating 14 is disposed to receive light from the slit and focus the same into a pair of flow cells; one such cell 15 is shown in the drawing, the other being above or below the plane of the drawing. Orientation of the grating about a vertical axis 16 at the central reflection point determines a wavelength incident on both of the cells. End windows 17 retain flowing liquid in the cell bores. A photodetector 18 after each cell detects the amount of light transmitted. One cell is used as a standard for comparison of the other receiving sample injected into the liquid.
U.S. Pat. No. 4,886,356 of the present assignee discloses a detector cell assembly useful for liquid chromatography. A problem suggested in the patent is effect of transients due to temperature changes causing non-uniform index of refraction of the liquid in the cell. Thermal insulation offered by the patent disclosure, or in other cases a heat sink, alleviate the temperature effects; however similar effects may result when test sample injected into the liquid reaches the cell, before equilibrium is reached. Small size of such cells is directed to minimizing such effects but has become insufficient for improved sensitivity and accuracy.
U.S. Pat. No. 4,011,451 discloses a lens for directing beams of light onto a pair of cells which are diverging so as to keep aberrant light from the walls. U.S. Pat. No. 4,037,974 discloses various methods for blocking light reflected from or through the cell walls. U.S. Pat. No. 3,792,929 discloses a field lens for tunneling radiation through the cell without striking the walls. U.K. Patent Application GB 2 116 707 A is directed to minimizing boundary effects in the cell with the use of a field stop and an aperature stop in the optical train directing light into the cell.